This invention relates to detector array systems and more particularly to an array of detectors which tends to have a fixed pattern noise in addition to random noise associated with signal outputs from the detectors.
There is an ever-increasing interest in solid-state imaging systems. These solid-state systems eliminate the need for a target electrode in conventional vidicon tubes, thus increasing basic resolution and speed capability. Upon being exposed to a light image, the typical photodetector collects the released photocharge in a p-n junction capacitance. The charge pattern can be read out without a scanning electron beam. Advantages are higher geometric accuracy, high sensitivity, higher scan rates, small size, low voltage, low power and solid-state ruggedness and reliability. Electronic circuits needed to scan the array can be formed on the silicon wafer while the array of p-n junction photodetectors is being formed using the most advanced integrated circuit technology. Such arrays are commercially available in both linear and rectangular configurations.
A typical array consists of p-n junction diodes produced in a silicon wafer as an integrated circuit with a quartz window. Each diode has inherent capacitance and an area which varies by as much as five percent, or more, from diode to diode. Each diode is connected to an output video line by an access switch, made from an MOS field-effect transistor. A shift register is provided to sequentially turn on the access switches. In the case of a rectangular array, a second shift register can be employed to switch the output of the first shift register from one row of access switches to the next as the array is scanned row by row.
As each access switch is turned on, the inherent capacitance of its associated diode is recharged back to the video output line potential, thus replacing the charge displaced by the photocharge. The replaced charge is amplified by a charge (trans-impedance) amplifier. Once the addressing switch is again turned off, the diode capacitance will begin to discharge due to further photocurrent. The amount of discharge is proportional to the intensity of the light impinging the diode during the entire period before the access switch is again turned on. The resulting signal at the video output is a train of pulses, each pulse having an amplitude proportional to the integrated light flux impinging the diode.
The recording and/or display of the video output is conventional for such discrete sampling systems. Typically, a sample-and-hold circuit is employed to hold the amplitude of each pulse in succession to provide continuity from pulse to pulse. In practice, it is desirable to provide an integrator between the charge amplifier and the sample-and-hold circuit, and to then quantize the sample for digital control of the recording or display device.
Pulses driving the linear array addressing register are used to synchronize the recording or display device. In the case of a rectangular array, the output of a second register may be employed to synchronize the display or recording of successive rows. When all the rows have been displayed side by side, the entire cycle is repeated.
To provide for an area image with a linear array, the optics focusing the image onto the array are turned at just the proper rate to match up successive line images. This basic drive relationship is then used to generate signals for a display device, such as a cathode-ray tube or further to generate a speed command for a film recording system.
It has been found thus far that such solid-state imaging systems produce an image which is just comparable to conventional vidicon tubes. The reason is that the area under each video pulse is indicative of the amount of integrated photocurrent augmented by the random noise present in the diode and the clock noise introduced by switching the access switch on.
The switching noise is the most serious contributing factor to distortion of the video output because the MOS transistor has associated with it junction capacitance between the gate and source electrodes. Such capacitance will couple a subsstantial charge from the switching control signal into the video output line. Other switching devices, such as junction transistors, could have an even greater problem. There are circuit techniques which may be employed to decrease this error, but such techniques will only provide a first order correction. That is because such techniques rely on introducing a compensating charge, but the junction capacitance is not the same for every switch and diode combination so there will be some residual error in the compensation of most diodes. The combined effects of variations in the (1) switch capacitance and (2) clock noise of the diodes in the array present a fixed pattern noise problem.